Optical fibre curvature sensor and measurement device comprising said sensor

ABSTRACT

An optical fiber curvature sensor. Two networks (R1, R2) with periodic longitudinal modulation of the refractive index of the optical fiber core are inscribed in the fiber (F) one behind the other or one on top of the other. The networks are configured to respectively reflect wavelengths λ1 and λ2 such that λ1=λB+ΔλB1 and λ2=λB+ΔλB2, where λB is the Bragg wavelength of the networks and where λB1 and λB2 are shifts sensitive to the temperature, to deformations and to the curvature of the optical fiber. The two networks are defined so that the quantities ΔλB1 and ΔλB2 have substantially identical sensitivity to temperature and to deformations and substantially opposite sensitivity to curvature.

TECHNICAL FIELD

This invention relates to the field of measuring the curvature of a structure by means of an optical fiber. The invention relates more particularly to an optical fiber curvature sensor and a measuring device comprising the sensor. The invention has in particular applications in the field of energy for measuring the curvature of cables, such as submarine cables, in the field of robotics for measuring the curvature of robot arms or in the medical field for measuring the curvature of umbilical links of endoscopes.

BACKGROUND ART

A certain number of optical fiber curvature sensors are known. The sensor is placed on the element of which the radius of curvature is to be measured in such a way that the optical fiber of the sensor hugs the shape of the element. Measuring the radius of curvature then reverts to measuring the radius of curvature of the optical fiber.

Certain optical fiber sensors are based on the creating of a fault zone in the optical fiber generally obtained by polishing the sheath. This zone induces losses in intensity when it is curved, losses which depend on the radius of curvature. Determining losses then makes it possible to determine the radius of curvature of the optical fiber.

Another technique consists in measuring intermodal interferences taking advantage of the sensitivity of cladding modes to the curvature. This technique however requires complex architectures, namely a micro-structured fiber inscribed in a drawn zone welded between two optical fibers.

Finally, other methods are based on the use of fiber index gratings such as a Bragg grating or a long period grating. These two types of gratings are gratings inscribed in the optical fiber. They differ by their pitch which is of a few hundred nanometres for a Bragg grating and of a few tens or even a few hundred micrometres for a long period grating.

A Bragg grating sensor is shown in FIG. 1. The Bragg grating R is inscribed in the core of the optical fiber. The Bragg grating reflects a specific frequency, called Bragg wavelength λ_(B), and transmits all of the other frequencies.

This Bragg wavelength λ_(B) is proportional to the pitch of the grating (Λ) and to the effective index of the core of the fiber (n_(eff)): λ_(B)=2·n _(eff)·Λ  (1)

Any modification of one of these parameters proportionately displaces the Bragg wavelength.

Given that the Bragg wavelength depends on the pitch of the grating (Λ), fiber index gratings can therefore be manufactured to reflect different Bragg wavelengths.

The variations in the stress applied to the fiber and the variations in the temperature of the fiber affect both the effective refractive index n_(eff) and the pitch Λ of the fiber index grating, which results in a shift Δλ of the reflected wavelength. The term stress means any type of force applied to the optical fiber, such as a force of torsion, compression, tension or curvature.

The shift in the wavelength Δλ of the light reflected with respect to the Bragg wavelength λ_(B) therefore depends on the curvature of the optical fiber but also on the temperature of the fiber and on the other stresses applied to the optical fiber. This shift in the wavelength therefore does not supply a direct measurement of the radius of curvature of the optical fiber.

SUMMARY

An object of embodiments of the invention is to overcome all or a portion of the disadvantages of the aforementioned prior art.

More particularly, an object of embodiments of the invention is to propose an optical fiber curvature sensor that uses the technique of fiber index gratings but which makes it possible to directly determine the radius of curvature from wavelengths.

Another object of embodiments of the invention is to propose a curvature sensor that is simple to produce and which is of small size.

To this effect, embodiments of the invention propose a curvature sensor comprising

-   -   at least one optical fiber comprising a core and at least one         first sheath surrounding the core, the core and the at least one         first sheath having different refractive indexes, the at least         one optical fiber further comprising an end for receiving         polychromatic light,     -   a first grating with periodic longitudinal modulation of the         refractive index of the optical fiber core, called first         grating, inscribed in the core of the at least one optical fiber         and configured to reflect a wavelength λ₁ of the light, the         wavelength λ₁ being shifted by a quantity Δλ_(B1) with respect         to a reference wavelength λ_(B) and the quantity Δλ_(B1) being         sensitive to the temperature, to deformations and to the         curvature of the optical fiber,     -   a second grating with periodic longitudinal modulation of the         refractive index of the optical fiber core, called second         grating, inscribed in the core of the at least one optical fiber         and configured to reflect a wavelength λ₂ of the light, the         wavelength λ₂ being shifted by a quantity Δλ_(B2) with respect         to the reference wavelength λ_(B) and the quantity Δλ_(B2) being         sensitive to the temperature, to deformations and to the         curvature of the optical fiber,     -   the first and second gratings being defined so that the         quantities Δλ_(B1) and Δλ_(B2) have substantially identical         sensitivities to temperature and to deformations and         substantially opposite sensitivities to curvature.

According to embodiments of the invention, we have: α_(T1)=α_(T2), α_(ε1)=α_(ε2) and f₂(R)=−f₁(R) if α_(t1) and α_(T2) designate respectively the sensitivity of the first grating to the temperature and the sensitivity of the second grating to the temperature, α_(ε1) and α_(ε2) designate respectively the sensitivity to deformation of the first grating and the sensitivity to deformation of the second grating and f1(R) and f2(R) designate respectively the shift in the wavelength due to the curvature in the first grating and the shift in the wavelength due to the curvature in the second grating.

The sensor of the invention therefore delivers reflected wavelengths λ₁=λ_(B)+Δλ_(B1) and λ₂=λ_(B)+Δλ_(B2). The two gratings reflecting these wavelengths having identical behaviors with respect to the temperature and to deformations but opposite with respect to the curvature, when these two wavelengths are subtracted, the difference Δλ=λ₁−λ₂=Δλ_(B1)−Δλ_(B2) depends solely on the curvature of the fiber. The curvature of the gratings can as such be deduced directly from the difference Δλ.

According to a particular embodiment, the sensor comprises a single optical fiber and the first and second gratings are Bragg gratings inscribed one behind the other in the core of the optical fiber, said first and second gratings having different average effective indexes.

According to another particular embodiment, the sensor comprises a single optical fiber and the first and second gratings are inscribed one on top of the other, the first grating being a Bragg grating and the second grating being a long period grating. In this embodiment, the optical fiber comprises advantageously a second sheath surrounding the first sheath, the second sheath having a refractive index less than the refractive index of the first sheath.

According to another embodiment, the sensor comprises a single optical fiber and a plurality of Bragg gratings inscribed one behind the other in the core of the optical fiber, the plurality of Bragg gratings being arranged in such a way as to behave as the association of a Bragg grating and a long period grating. More particularly, the sensor comprises a superstructured Bragg grating, commonly referred to as SFBG (Superstructured Fiber Bragg Grating). To carry out this superstructured grating, a hundred or so very short Bragg gratings in series are inscribed in the core of the fiber. All of the Bragg gratings are identical (same pitch, same length, same index modulation), The gratings are regularly spaced by a distance L_(LPG). Their length L_(FBG) is a fraction of L_(LPG). The total length of the structure is about a centimeter, as with a conventional grating. This structure behaves as the association of a Bragg grating with pitch L_(FBG) and of a long period grating with pitch L_(LPG).

According to another particular embodiment, the sensor comprises first and second optical fibers in a resin bar having an axis of symmetry, the first and second gratings being inscribed respectively in the first and second optical fibers. In this embodiment, the first and second gratings are advantageously inscribed at substantially identical positions along the axis of symmetry and the first and second optical fibers are placed at equal distances from the axis of symmetry.

In this embodiment with two optical fibers, the first and second gratings can be Bragg gratings. According to an alternative, the first and second gratings are long period gratings.

Embodiments of the invention also relate to a device for measuring the curvature of a longitudinal element, characterized in that it comprises:

-   -   a curvature sensor such as defined hereinabove, the at least one         optical fiber of the curvature sensor being arranged along the         element,     -   a source of polychromatic light for emitting light through the         at least one optical fiber, and     -   a circuit for receiving the wavelengths λ₁=λ_(B)+Δλ_(B1) and         λ₂=λ_(B)+Δλ_(B2) and determining the curvature of the element         from the wavelengths.

This device makes it possible to deliver in a simple manner a value of the curvature of the longitudinal element.

Other advantages can further appear to those skilled in the art when reading the examples hereinbelow, shown in the accompanying figures, given for the purposes of information.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 diagrammatically shows a curvature sensor from prior art;

FIG. 2 is a diagram showing the change in the shift of the Bragg length Δλ_(B) according to the radius of curvature R for three average grating effective index values δn_(dc);

FIG. 3 diagrammatically shows a curvature sensor in accordance with an embodiment of the invention;

FIG. 4 shows a cross-section view of an optical fiber of the curvature sensor of an embodiment of the invention;

FIG. 5 shows the refractive index profile of the optical fiber of FIG. 4;

FIG. 6 is a diagram showing the change in the shifts Δλ_(B1) and Δλ_(B2) of the Bragg lengths of the two gratings of the curvature sensor of FIG. 3 according to the curvature 1/R;

FIG. 7 is a diagram showing the change in the shift Δλ=Δλ_(B1)−Δλ_(B2) according to the curvature 1/R;

FIG. 8 diagrammatically shows a device for measuring the curvature comprising the sensor of FIG. 3.

DETAILED DESCRIPTION

Embodiments of the invention are based on the fact that the variation in the resonant length (or Bragg wavelength λ_(B)) of an index grating such as a Bragg grating or a long period grating is governed by the average effective index n_(eff) of the grating.

When such a grating is curved, the resonance wavelength is shifted. The shift is given by: Δλ_(B)=2·(Δn _(eff) +δn _(dc)·Δκ_(eff))·Λ  (2) where Λ is the pitch of the grating, Δn_(eff) is the variation in the effective index n_(eff) of the core of the fiber due to the curvature, δn_(dc) is the average effective index of the grating and Δκ_(eff) is the variation in the coupling coefficient κ_(eff) of the grating due to the curvature. These two factors depend only on the optical fiber and change in opposite directions: n_(eff) increases when the radius of curvature decreases while κ_(eff) decreases when the radius of curvature decreases. It can be seen in the relationship (2) that the variation in the coupling coefficient Δκ_(eff) is multiplied by the average effective index δn_(dc). Therefore, according to this parameter, the variation in the coupling coefficient Δκ_(eff) can either be negligible compared to the variation in the effective index Δn_(eff), or offset it or be much greater than the latter. It can be deduced from the above that the variation in the resonance wavelength Δλ_(B) can be either negative, or zero or positive, such as is shown in FIG. 2 for a Bragg grating. By varying the average effective index δn_(dc) of the grating, it is possible to set the shift in the wavelength Δλ_(B) to a negative, zero or positive value for a given radius of curvature R.

In the example of FIG. 2, a shift Δλ_(B) is obtained according to the radius of curvature R which is:

-   -   positive for δn_(dc)=5.10⁻⁴;     -   zero for δn_(dc)=1.62.10⁻³;     -   negative for δn_(dc)=5.10⁻³.

The idea of embodiments of the invention is therefore to associate two index gratings having the same sensitivity to deformation and to the temperature hut opposite sensitivities according to the curvature.

According to embodiments of the invention, the sensor proposed therefore comprises two fiber index gratings having the same sensitivity to the temperature and to deformations but opposite responses according to the radius of curvature. A block diagram of this sensor is shown in FIG. 3.

In reference to FIG. 3, the sensor comprises two Bragg gratings R1 and R2 arranged in series on an optical fiber F. The two gratings are photo-inscribed in the core of the optical fiber. As these two gratings are made from the same material, they have the same sensitivity to the temperature. The two gratings R1 and R2 are also designed in such a way as to have the same sensitivity to deformations (torsion, compression, tension or elongation) and opposite responses to the curvature.

As such, subjected to the same conditions of temperature, of deformation and of curvature, the two sensors R1 and R2 react in the following way:

$\begin{matrix} \left\{ \begin{matrix} {{\Delta\lambda}_{B\; 1} = {{\alpha_{T}T} + {\alpha_{ɛ}ɛ} + {f(R)}}} \\ {{\Delta\lambda}_{B\; 2} = {{\alpha_{T}T} + {\alpha_{ɛ}ɛ} - {f(R)}}} \end{matrix} \right. & (3) \end{matrix}$ where Δλ_(B1) is the variation in the wavelength of the grating R1, Δλ_(B1) is the variation in the wavelength of the grating R2, T is the temperature of the optical fiber, α_(T) is the sensitivity of the grating to the temperature, ε represents the deformation of the fiber, α_(ε) is the sensitivity to deformation, +f(R) designates the shift in the wavelength due to the curvature in the grating R1 and −f(R) designates the shift in the wavelength due to the curvature in the grating R2.

When the sensor is subjected to a polychromatic light, the grating R1 reflects a light that has a wavelength λ₁=λ₂+Δλ_(B1) and the grating R2 reflects a light that has a wavelength λ₂=λ_(B)+Δλ_(B2).

If the reflected wavelengths λ₁ and λ₂ are subtracted, we obtain a magnitude Δλ that is independent of the temperature and of the deformations and which depends only on the radius of curvature R: Δλ=λ₁−λ₂=λ_(B)+Δλ_(B1)−λ_(B)−Δ_(B2)=Δλ_(B1)−Δλ_(B2) =f(R).

It is therefore possible to directly obtain the radius of curvature R from the shift in the wavelength Δλ.

The optical fiber F is a single-mode and step-index fiber that has the following characteristics:

-   -   radius of the core: a₁=4.2 μm;     -   index of the core of the fiber: n₁;     -   outer radius of the sheath: a₂=62.5 μm;     -   index of the core of the fiber: n₂.

The dimensions and the index profile of the optical fiber can be seen in FIGS. 4 and 5.

The index of the sheath n₂ is evaluated from the Sellmeier relationship applied to the silica:

$\begin{matrix} {{n^{2}(\lambda)} = {A + \frac{B}{1 - \frac{C}{\lambda^{2}}} + \frac{D}{1 - \frac{E}{\lambda^{2}}}}} & (4) \end{matrix}$ where A, B, C, D and E are the Sellmeier coefficients that depend on the temperature via the relationship X=aT+b, with T the temperature expressed in degrees centigrade.

The coefficients a and b of Sellmeier A, B, C, D and E of the silica are expressed in the following table:

Coefficient X = aT + b a b A 6.90754 · 10⁻⁶ 1.31552 B 2.35835 · 10⁻⁵ 0.788404 C 5.84758 · 10⁻⁷ 1.10199 · 10⁻² D 5.48368 · 10⁻⁷ 0.91316 E 100 0

The index of the core n₁ is deduced from the index of the sheath n₂ by the relationship: n₁=1.0036 n₂.

The grating R1 has a length L1=8.9 mm, a grating pitch Λ₁=541.1 nm and an average effective index δn_(dc1)=1·10⁻⁴·n₁. The grating R2 has a length L2=250 μm (micrometers), a grating pitch Λ₂=541.4 nm and an average effective index δn_(dc2)=3.5·10⁻³·n₁.

The wavelengths λ₁ and λ₂ reflected respectively by the gratings R1 and R2 (at rest) are then:

λ₁=1565.2 nm and λ₂=1570 nm.

These resonant wavelengths are sufficiently spaced to prevent any superposition of the resonances or inversion in their position in the curvature range 1/R∈[0; 1] cm⁻¹.

As can be seen in FIG. 6, the shift in the wavelength Δλ_(B1) of the grating R1 decreases with the curvature of the fiber while the shift in the wavelength Δλ_(B2) of the grating R2 follows an opposite curve.

The sensitivity to axial deformation (α_(ε)) of the grating R1 is identical to that of the grating R2 and is evaluated at 1.23 pm/με (where 1 με corresponds to a deformation of 10⁻⁶ m/m). Likewise, the sensitivities to the temperature (α_(T)) of the two gratings R1 and R2 are substantially identical, of about 12.02 pm/° C.^(cent).

This results in that the subtraction of the two signals of wavelength λ₁ and λ₂, i.e. Δλ=λ₁−λ₂=Δλ_(B1)−Δλ_(B2), is independent of the temperature T and of the deformations ε and depends solely on the curvature of the optical fiber. The curve of FIG. 7 shows the dependency between shift in the wavelength Δλ and the curvature of the sensor of FIG. 3, The dependency is non-linear. Simply measuring the shift Δλ makes it possible to obtain the radius of curvature from this curve.

In the embodiment shown hereinabove, the gratings R1 and R2 are Bragg gratings inscribed one behind the other in the optical fiber F. As indicated hereinabove, these two gratings differ only by their average effective indexes (δn_(dc1) and δn_(dc2)), their pitches (Λ₁ and Λ₂) and their lengths (L₁ and L₂) in such a way that their dependencies on the curvature are opposite.

According to an alternative embodiment, the gratings R1 and R2 are respectively a Bragg grating and a long period grating inscribed in the core of the optical fiber F one on top of the other. The optical fiber F comprises advantageously two sheaths. The second sheath is used to insulate the light that propagates in the first sheath of the outer medium. Its refractive index is less than that of the first sheath. The two gratings advantageously have the same length. The long period grating is designed in such a way as to have only a resonance in the measured spectral range. Moreover, the resonant mode is chosen so as to have the same sensitivity to deformation as the Bragg grating. The average effective indexes of the two gratings are such that the responses of the two gratings to the curvatures are opposite.

According to another embodiment, the sensor comprises a single optical fiber and a plurality of Bragg gratings inscribed one behind the other in the core of the optical fiber, the plurality of Bragg gratings being arranged in such a way as to behave as the association of a Bragg grating and a long period grating.

More particularly, the sensor comprises a superstructured Bragg grating, commonly referred to as SFBG for Superstructured Fiber Bragg Grating. In order to produce this superstructured grating, a hundred or so very short Bragg gratings in series are inscribed in the core of the fiber. All of the Bragg gratings are identical (same pitch, same length, same index modulation). The gratings are regularly spaced by a distance L_(LPG). Their length L_(FBG) is a fraction of L_(LPG). The total length of the structure is about one centimeter, as with a conventional grating. This structure behaves as the association of a Bragg grating with a pitch L_(FBG) and of a long period grating with a pitch L_(LPG).

According to another alternative embodiment, the sensor comprises two optical fibers arranged in a resin bar having an axis of symmetry. The two fibers are advantageously placed at equal distances from the axis of symmetry of the bar. The grating R1 is inscribed in the first fiber and the second grating is inscribed in the second fiber. They are advantageously inscribed at substantially identical positions along the axis of symmetry. In this embodiment, the gratings R1 and R2 can be Bragg gratings or long period gratings. In this latter case, the optical fibers advantageously comprise two sheaths. As with the other embodiments, the average effective indexes of the two gratings are selected so that the responses of the two gratings to the curvatures are opposite.

As explained hereinabove, the wavelengths λ₁ and λ₂ coming from the sensor make it possible to determine the radius of curvature. These two wavelengths must therefore be received and processed in order to obtain the radius of curvature. The invention therefore relates to, more globally, a device for measuring the curvature of a longitudinal element comprising:

-   -   a curvature sensor according to one of the embodiments described         hereinabove, with the optical fiber of the curvature sensor         being arranged along the element of which the radius of         curvature is to be measured,     -   a source of polychromatic light in order to emit light through         the optical fiber of the sensor, and     -   a circuit for receiving the wavelengths λ₁=λ_(B)+Δλ_(B1) and         λ₂=λ_(B)+Δλ_(B2) coming from the curvature sensor and         determining the curvature of the element from said wavelengths.

Such a device is shown diagrammatically in FIG. 8. It comprises a source of white or polychromatic light 10, a curvature sensor 11 such as defined hereinabove for receiving the light emitted by the source 10 and delivering wavelengths λ₁ and λ₂ corresponding to the wavelengths reflected by the gratings R1 and R2 of the sensor, a circuit 12 for determining the radius of curvature R of the element from the wavelengths λ₁ and λ₂. A coupler 13 is used for the transmission of the polychromatic light from the source 10 to the sensor 11 and the transmission of the wavelengths λ₁ and λ₂ from the sensor 11 to the circuit 12. The circuit is for example an interferometer equipped with means for processing in order to perform the subtraction Δλ=λ₁−λ₂ and to deduce therefrom, for example by means of a look-up table, the radius of curvature R of the element.

Of course, it is possible to arrange several curvature sensors in accordance with embodiments of the invention along the element, with offset resonant wavelengths, in order to measure the curvature at several points of the latter.

The sensor and the device presented here have many advantages:

-   -   easy to manufacture;     -   easy to implement,     -   reduced size of the sensor;     -   obtaining of the radius of curvature directly from a difference         in wavelength;     -   insensitivity to the drops in intensity of the light emitted or         reflected.

Moreover, as the measurement proposed is independent of the temperature and of the deformation, the invention can be used in many fields, in particular in applications where the sensor can be subjected to temperature gradients, for example in maritime or medical applications.

Embodiments of the invention are described in the above by way of example. It is understood that those skilled in the art are able to produce various alternative embodiments of the invention, by associating for example the various characteristics hereinabove taken alone or in combination, without however leaving the scope of the invention. 

The invention claimed is:
 1. Curvature sensor comprising: at least one optical fiber (F) comprising a core and at least one first sheath surrounding said core, said core and said at least one first sheath having different refractive indexes, said at least one optical fiber further comprising an end for receiving polychromatic light, a first grating for periodic longitudinal modulation of the refractive index of the optical fiber core, called first grating (R1), inscribed in the core of said at least one optical fiber and configured to reflect a wavelength λ₁ of the light, said wavelength λ₁ being shifted by a quantity Δλ_(B1) with respect to a reference wavelength λ_(B) and said quantity Δλ_(B1) being sensitive to the temperature, to deformations and to the curvature of the optical fiber, at least one second grating for periodic longitudinal modulation of the refractive index of the optical fiber core, called second grating (R2), inscribed in the core of said at least one optical fiber and configured to reflect a wavelength λ₂ of the light, said wavelength λ₂ being shifted by a quantity Δλ_(B2) with respect to said reference wavelength λ_(B) and said quantity Δλ_(B2) being sensitive to the temperature, to deformations and to the curvature of the optical fiber, wherein the average effective indexes, pitches and lengths of first and second gratings are configured such that the quantities Δλ_(B1) and Δλ_(B2) have substantially identical sensitivities to temperature and to deformations and substantially opposite sensitivities to curvature and such that the difference between λ₁ and λ₂ is substantially independent of the temperature and deformations and depends on a radius of the curvature of the optical fiber.
 2. Sensor according to claim 1, further comprising a single optical fiber, the first and second gratings (R1, R2) being Bragg gratings inscribed one behind the other in the core of the optical fiber, said first and second gratings having different average effective indexes (δn_(dc1),δn_(dc2)).
 3. Sensor according to claim 1, further comprising a single optical fiber and in that the first and second gratings (R1, R2) are inscribed one on top of the other, the first grating being a Bragg grating and the second grating being a long period grating, said first and second gratings having different average effective indexes (δn_(dc1),δn_(dc2)).
 4. Sensor according to claim 3, wherein the optical fiber comprises a second sheath surrounding said first sheath, said second sheath having a refractive index less than the refractive index of the first sheath.
 5. Sensor according to claim 1, comprising a single optical fiber and a plurality of Bragg gratings inscribed one behind the other in the core of the optical fiber, said plurality of Bragg gratings being arranged in such a way as to behave as the association of a Bragg grating and a long period grating.
 6. Sensor according to claim 1, comprising first and second optical fibers in a resin bar having an axis of symmetry, the first and second gratings being inscribed respectively in said first and second optical fibers.
 7. Sensor according to claim 6, wherein the first and second gratings (R1, R2) are arranged at substantially identical positions along said axis of symmetry and said first and second optical fibers are placed at equal distances from said axis of symmetry.
 8. Sensor according to claim 6, wherein said first and second gratings (R1, R2) are Bragg gratings.
 9. Sensor according to claim 6 or, wherein said first and second gratings (R1, R2) are long period gratings.
 10. Device for measuring the curvature of a longitudinal element, comprising: a curvature sensor according to claim 1, said at least one optical fiber of the curvature sensor being arranged along said element, a source of polychromatic light for emitting light through said at least one optical fiber, and a circuit (12) for receiving the wavelengths λ₁=λ_(B)+Δλ_(B1) and λ₂=λ_(B)+Δλ_(B2) and determining the curvature of said element from the difference between λ₁=λ_(B)+Δλ_(B1) and λ₂=Δ_(B)+Δλ_(B2). 